You are not alone! Thousands have suffered the same fate before you. This is one of the well known bug that the game comes with. It can occur randomly for any mission. Here's what CM's OFP FAQ page has to say about this:
Problem: When playing a mission in the campaign, the campaign does not continue, the game just goes back to the main menu. This is even after the mission was successfully completed.
Solution: Replaying a campaign mission will result in replaying that mission as a single mission. You should use the 'Revert' option when replaying a mission from the campaign. If this option is not available, you will have to select 'Revert' on a previous mission. If you have received a debriefing from Colonel Blake, this means that you have failed the campaign. You will need to revert from an earlier mission and get a higher individual score on the missions.
If the problem persists, despite having tried the revert solution, do the following:
1.Load up Windows Explorer
2.Go to the folder [install directory]\OperationFlashpoint\users\[username]\saved\campaigns\1985. (The default install directory is C:\Program files\Codemasters and the [username] folder will be the user name you are playing with.
3.If there are subfolders from the 1985 folder (e.g. ..\missions\00Training.Abel) you should copy all the *.fps files to the 1985 folder.
4.You should now be able to proceed in the campaign.
If you've tried that and you still cannot continue in the campaign or you simply do not want to revert to the last campaign mission you played and do it all over again, you can replace your copy of OFP's 1985.SQC file.